Exhaust manifolds are commonly made from cast iron for high volume production engines because for this application cast iron often has advantages in terms of cost, durability, packaging and NVH (noise, vibration, and harshness). Packaging refers to the task of arranging flow paths from each port to a common outlet position while maintaining clearance to other underhood components and providing access for all fasteners during assembly. Among the commonly used cast iron materials for exhaust manifolds is silicon-molybdenum cast iron (“SiMo cast iron”). SiMo cast iron becomes weaker as the temperature increases and is subject to damage from oxidation, decarburization, and coarsening at very high temperatures. The duration of time at high temperature determines the amount of material damage that accumulates. The accumulation of damage and the elevated temperature strength (the thermal strength) of the material are important factors in evaluating the durability of the exhaust component.
As automotive companies increase the gas temperatures of their engines to improve efficiency and reduce exhaust emissions, more manifold applications are exceeding the practical working (temperature) limit of typical cast irons. The temperature distribution in the manifolds is not uniform and some peak temperature areas receive more heat than other areas in the manifolds. Currently, if a material such as SiMo cast iron is inadequate for the peak temperature, the entire manifold has to be made from a higher grade material (e.g., Ni-Resist, cast steel, or fabricated stainless steel). Therefore, the manufacturing costs for exhaust manifolds for high temperature applications are significantly increased.